All rotating machinery has bearings that eliminate friction between rotor and the stationary housing (casing). These bearings also support loading of the rotor. The primary bearing types are hydrodynamic bearings that support high speed machinery with speeds over 4000 rpm, and anti-friction bearings supporting machinery with speeds below 4000 rpm.
Within the anti-friction bearing category, there are ball, cylindrical, tapered, roller and spherical bearings. The ball bearing consists of the inner race, outer race, cage, and balls. These balls may be single row or double rows according to loads.
The inner race is fitted to the rotating shaft and spins with it. The outer race is fitted to the bearing housing and remains stationary. The balls are placed inside the cage between the inner and outer race and have a relative motion between them. For safe rotation without wear or friction, the balls should be properly lubricated to protect them from wear.
Ball lubrication should be either oil or grease according to load, speed, working temperature and bearing design. Bearing design should be either self-lubricated or manually lubricated.
The self-lubricated bearing has grease sealed inside the cage to avoid leakage. But when the grease reaches its lifetime limit its viscosity decreases due to its conversion from high viscous mode to liquid mode. At this point, friction and wear begin to affect rotating balls leading to vibration. This, in turn, leads to leakage and bearing damage. At end of life, therefore, bearings should always be changed out in order to safeguard other rotating components.
For bearings with manual lubrication, grease has to be added periodically via a special hole. Late greasing can cause friction and wear, thereby damaging the balls. Use the grease recommended by the manufacturer to ensure it has stable viscosity at high temperature. Poor-grade grease can convert to liquid quickly and damage the bearing.
Greater safety and protection
For greater safety, the greasing tip can be covered by grease to protect it from rust, if conditions around it are poor. For example, the presence of cooling tower near the machinery can lead to water droplets falling continually on pumps and motors.
In particular, watch out for the surroundings of the motor legs, which are mounted with bolts to the concrete foundation. Shims can sometimes be placed below these legs to adjust alignment between the motor and the pump and to overcome the effect of soft foot that can lead to excess vibration. But if these legs have no cover or coating, corrosive conditions can cause rust formation which then creates soft foot. Another cure is to build a room or a box around the pump and motor which can be removed during maintenance.